1. Field of the Invention
The disclosed embodiments of the present invention relate to a calibration method and a related calibration circuit, and more particularly, to a calibration method and a related calibration apparatus for calibrating mismatches of an in-phase (I) signal path and a quadrature signal (Q) path of a transmitter/receiver.
2. Description of the Prior Art
In general, the modulation technique can comprise more information for data transmission when it is more complicated. That is, the modulation technique can increase transmission rate by complicated modulation process, such as the 64-Quadrature Amplitude Modulation (64-QAM), or even the 256-QAM. Thus, the requirement of high level modulation techniques is more and more popular. If a higher modulation scheme is used for delivering more information data, the Error Vector Magnitude (EVM) of the communication system has to be lower accordingly. One of the most important factors affecting the EVM is the level of an imbalance between the In-phase and Quadrature-phase signal paths (IQ imbalance) in modern communication transceivers. The main cause of the IQ imbalance comes from the circuit mismatch of IQ signal path in a Radio Frequency (RF) communication system. Even a small bias will affect the whole communication system, to form an incomplete orthogonal modulation/demodulation procedure, and result in increase of Bit Error Rate (BER) of the receiving terminal. The mismatch can be separated to the amplitude mismatch and the phase mismatch. As long as the mismatches exist, an image interference would appear at the symmetrical frequency of the original signal in the spectrum. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a received signal received by a receiving terminal and an undesired image signal accompanying the received signal due to mismatch effect. The difference between the received signal amplitude and the image signal amplitude is called Image Rejection Ratio (IRR) in general. For example, when IQ imbalance is large, IRR will be small, and IQ imbalance is small, IRR will be large.
In order to improve the mismatch effect, a calibration will be performed for the circuit before transmitting/receiving signals formally, and the calibration is called IQ calibration. There are two main causes of the mismatches between the I and Q signal paths. One of the main causes is when a local oscillator (LO) generates carrier waves and transmits the carrier waves respectively to the mixers in the I signal path and the Q signal path, it is very hard to have a phase difference of exact 90 degrees, or the amplitudes of the carrier waves respectively transmitted to the mixers in the I signal path and the Q signal path are different (i.e., the above phase mismatch and amplitude mismatch). Another one of the main causes is that it is hard to avoid imperfect conditions in the circuit process, and thus two elements in the I signal path and the Q signal path are not totally matched to each other. For example, when there are mismatches between the low-pass filter (LPF) pair, the analog-to-digital converter (ADC) pair, the digital-to-analog converter (DAC) pair, or the gain amplifier in the I signal path and the Q signal path, the signals passing by the I signal path and the Q signal path will have difference. When the signals pass by the I and Q signal paths with mismatches, the image interference will be generated (as shown in FIG. 1) and reduce the signal quality.
In general, the prior art uses the searching method to approximate the optimal compensating value gradually to calibrate the image interference signals. However, the requirement for connecting speed is higher and higher in the wireless communication application nowadays. For example, if there is a phone call coming when a user uses a Bluetooth earphone, the Bluetooth earphone has to connect to the smartphone right away for user to take the phone call. The higher connecting speed can bring a better user experience. Thus, it is an important issue about how to perform the IQ calibration fast and accurately in the area of the communication system.
In addition, when the isolation between the local oscillator and radio-frequency circuit output is not perfect, the local oscillator signal leakage will occur to interfere the transmitting signals. In general, the prior art also uses the search-like method gradually tuning the parameters of compensation circuit to an optimal result to remove the local oscillator signal leakage. However, this method also has the disadvantage of slow speed. Thus, it is an important issue about how to compensate the local oscillator signal leakage fast and accurately in the area of the communication system.